Long distance optical fiber transmission systems, such as those employed in undersea or transcontinental terrestrial lightwave systems that use optical amplifiers, are subject to polarization mode dispersion (PMD) which causes serious signal degradation. PMD is caused by a difference in the group delay for two orthogonal modes of an optical signal propagating in an optical fiber. PMD is especially important in lightwave systems using optical amplifiers because, in such systems, the transmitted optical signal propagates along the entire lightwave system without retiming or regeneration. Over a several thousand kilometer lightwave system, PMD can cause the difference in group delay for the two orthogonal modes to be large enough to cause excessive pulse broadening which results in severe signal degradation.
Unlike other optical parameters, PMD is very sensitive to fiber configuration and environmental changes such as temperature and pressure. For example, a fiber laid in a straight path on a smooth surface has higher PMD than does a fiber which is tightly wound on bobbin. PMD also increases in optical fibers during the cabling process because fibers are cabled in a purposefully relaxed condition where stress and strain on the fibers is minimized. PMD is higher in optical fibers in a relaxed condition because there is less of the mode coupling of the orthogonal modes that results from random localized birefringence as when the fiber has perturbations due to stress or strain.
Although pertubations in optical fibers tend to reduce PMD, other fiber parameters can be adversely affected when the fiber is subjected to a high level of pertubations. For example, perturbations which reduce fiber relaxation and PMD tend to increase fiber loss and decrease fiber strength--a result which is contrary to present lightwave systems design rules. This most probably explains why most fiber optic cables are unsuitable for use in long distance lightwave transmission systems using optical amplifiers where low PMD is desired.